Extruders of molten resin are available in many forms and for many purposes, e.g. extrusion of thermoplastic resin into extruded shapes such as tubes, coatings, or rods, extrusion of polymer for blown film, single screw and twin screw extruders, and injection molding. The terms extruders and extrusion used herein includes all these types of melt extrusion operations and apparatus, including the extrusion carried out in the course of injection molding. Such extruders have a heated extrusion barrel and one or two screws revolving within the barrel to compress, melt and extrude the resin melt through an orifice in the extrusion nozzle. When the extruder has to switch from one resin to a different resin or resin of a different color, the preceding resin contaminates the succeeding resin. Impurities created during the extrusion of the preceding resin, such as carbon residue which would form black specs or other contaminants may also be present, depending on the length of extrusion run on the preceding resin and its tendency to decompose under prolonged heating. The creation of such impurities in the resin can become a problem even without product changeover, i.e. affecting the quality of the product made from the original resin. Similarly, the start-up of an extruder without change of resin from the previous run can lead to contaminants present in the extruder adversely affecting the resultant product, whether an extrudate or an injection molding.
Purging compounds have been used with moderate success to clean out the extruder. In the case of product changeover, the purging compound is run through the extruder after extrusion of the preceding resin is finished, to clean out the extruder of the preceding resin, and this is followed by extrusion of the succeeding resin. The purging compound is used similarly to clean out contaminants formed during a long extrusion run, followed by introducing the same thermoplastic resin feed to the extruder, or to start up an extruder. Such purging compounds have included blends of polyamide/high density polyethylene (HDPE) for purging extruders of polyamide, low density polyethylene (LDPE) with additives, and polycarbonate, mixture of carboxylic acid, carbonate, and fatty acid derivatives available as CENPURGE.RTM. from Central Chemicals Co. Such purge compounds include polymeric materials and non-polymeric materials, but all are called purge compounds. Prior purging compounds have suffered from one or more of the following disadvantages: resin specificity, i.e. only work with certain preceding polymers, take too long to accomplish purging, become a source of contamination themselves of the succeeding resin, are too abrasive, causing undesired wear of extruder parts, and give off odor. Such compounds have also generally failed to purge the extruder of carbon residue from the extrusion of the preceding polymer. To purge the extruder of carbon residue adhered to the barrel and extrusion screw(s), the extruder typically has to be disassembled and the barrel and screw surfaces subjected to intense manual cleaning, including abrasion of the affected surfaces, followed by heating in the case of the screw to oxidize the carbon which may nevertheless remain after the intense manual cleaning. This is expensive in terms of extruder downtime, labor cost for the cleaning, the cost of an oven and its operation, and wear of the screw and barrel from the intense cleaning.
The need exists for a purging compound which is relatively universal in its applicability to extrusion of molten resin, inert so as not to react with the thermoplastic resin being purged, odor free, and economical to use, including to purge the carbon residue to eliminate the need for intense manual cleaning and heating of the screw.